Membrane switch and air bag assembly including it

Abstract: A membrane switch (10) including first and second substantially flexible electrical contacts (18) and a first layer (22) of dielectric material deposited between and on one surface of one of the first or second contacts. the first layer (22) is deposited in a pre-determined geometric pattern including a plurality of cells (30) defined by cell walls (32). The cell walls (32) are broken in pre-determined positions about their perimeter so that during and after actuation of the switch air within a cell (30) between the first and second contacts (18) can vent to one or more adjoining cells (30) to enable activation and prevent a vacuum from forming between the first and second contacts (18) thereby enabling the switch to break contact after being depressed. Such a switch (10) is particularly useful to overlie an air bag (44) mounted in the centre of a steering wheel and to operate a horn and other accessories.

Description: This invention relates generally to membrane switches, and more particularly to a membrane switch which can be utilized in an air bag assembly for an automobile. Membrane switches typically include two opposing surfaces, each of which include circuits thereon which, when engaged, complete the desired electrical circuit. To maintain the two opposing surfaces apart and the circuit in an open state, some type of insulating layer or spacer is disposed between the two substrates. An example of such a membrane switch is illustrated in U.S. Patent No. 4,382,165 which discloses two circuit sheets, one of which is adhered to a backer or stiffener board. Each sheet includes circuit patterns on opposing surfaces including circuit paths and circular contact pads thereon. An extremely thin non-conductive adhesive layer is applied between the circuit patterns which covers the circuit paths but includes apertures at the position of each circular contact pad. In order to prevent unintentional short circuiting from tolerance variations or sagging of the circuit sheets, a discontinuous layer of adhesive in the form of lines or dots is applied in the apertured areas. Such a switch, however, must be utilized with a backer board and cannot provide variable actuation forces since wide spacing between the lines or dots causes sagging while small spacing can prevent ready actuation of the switch. Additionally, since the adhesive provides a seal about the apertured areas, a vacuum can be created when the switch is depressed preventing release of the contacts. Another membrane switch is illustrated in U.S. Patent No. 4,602,135 which discloses two sheets having opposing surface where one surface includes switch leads and the other surface includes circuit completing material. A non-conductive spacer is positioned between the switch leads and the circuit completing material where the spacer is formed as a grid made up of squares. As with the above described patent, a vacuum frequently is created between the layers of the switch when depressed which prevents release and breaking of the circuit. Additionally, such a switch also requires a stiff backing or mounting surface to enable activation. Document WO/90/10204 discloses a membrane switch comprising first and second substantially flexible electrical contact means and a first layer of dielectric material deposited between and on one surface of one of said first or second contact means, said first layer being deposited in a pre-determined geometric pattern including a plurality of cells defined by cell walls, wherein the cell walls are broken in pre-determined positions about the perimeter of each cell so that, during and after actuation of the switch, air within any cell can vent to one or more adjoining cells. According to a first aspect of the present invention, such a membrane switch is characterised in that said geometric pattern includes a series of hexagons defining a plurality of cells where each of the six sides of the hexagons are broken proximate a mid-point thereof to enable activation and prevent a vacuum from forming between the first and second contacts thereby enabling the switch to break contact after being depressed. According to a second aspect of this invention, an air bag assembly comprises: housing means having at least one opening therethrough; air bag means mounted within said housing means; an outer cover member enclosing said at least one opening in said housing and securing said air bag means therein, said outer cover capable of enabling release of said air bag means through said opening in said housing when activated; and a membrane switch means in accordance with the first aspect of this invention, mounted between said air bag means and said outer cover which can be activated by a user through said outer cover, enable unimpeded deployment of said air bag means while preventing damage to said air bag means and injury to a user. A particular embodiment of a membrane switch and its use in an air bag assembly will now be described with reference to the accompanying drawings, in which:- Figure 1 is a perspective partially exploded view of the membrane switch of the invention illustrating the opposing flexible switch assemblies and the patterns of the dielectric spacer layers thereon; Figure 2 is a cross sectional view of the membrane switch of the invention taken along lines 2-2 of Figure 1 and in the direction indicated generally; Figure 3 is a cross sectional view of the membrane switch of the invention, similar to Figure 2, illustrating the switch in conjunction with a non-flexible substrate; Figure 4 is a top plan view of the membrane switch of the invention with a portion of the top flexible switch assembly broken away illustrating a low activation force pattern of the dielectric spacer layer of the invention; Figure 5 is a top plan view of the membrane switch of the invention, similar to Figure 4, illustrating a high activation force pattern of the dielectric spacer layer of the invention; Figure 6 is a cross sectional view of the membrane switch of the invention taken along lines 6-6 of Figure 4 and in the direction indicated generally, illustrating actuation of the switch by a user; Figure 7 is a cross sectional view of the membrane switch of the invention, similar to Figure 6, illustrating release of the switch; Figure 8 is a perspective view of the membrane switch of the invention illustrated in conjunction with a control panel or keyboard; Figure 9 is a perspective view of a steering wheel and air bag assembly having the membrane switch of the invention secured therein; and Figure 10 is a perspective exploded view of the steering wheel and air bag assembly of Figure 9. Referring to Figure 1, the membrane switch of the invention generally is designated by the reference numeral 10. The switch 10 is composed of first and second switch assemblies 12 and 14 which substantially are mirror images of each other and are secured in face to face contact about their perimeters by an adhesive 16. The switch 10 substantially is flexible and can be provided in any size and shape. The adhesive 16 provides a seal about the perimeter of the switch 10 to prevent infiltration of the elements between the two assemblies 12 and 14. As Figures 1 and 2 illustrate, each assembly 12 and 14 preferably includes three layers. The first layer is a flexible substrate 18, such as Ultem (Trade Mark), polyester or the like, and forms the outside surface of each assembly 12 and 14. The second layer is a conductive layer 20, such as a polymer like silver ink or similar conductive material. The third layer is a dielectric spacer layer 22 which is formed on top of the conductive layer 20 in a pre-determined pattern which will be described in detail below. If desired, the dielectric layer 22 may only be included on one of the assemblies 12 or 14. The second conductive layer 20 is deposited with a uniform thickness about an entire inside surface 24 of the flexible substrates 18, such as by deposition, silk screening or any similar process. To enable contact of the conductive layers 20 to the remainder of the electrical circuit, each flexible substrate 18 includes a tab 26 formed at a desired position about the perimeter of the substrate 18 where a contact portion or lead 28 of the conductive layer 20 is deposited. Although the switch 10 is illustrated as containing only one individual switch mechanism or area, it is to be understood that the single switch 10 can be broken up into one or more individual switches having their own tabs and contact portions (not illustrated). Such a switch readily can be utilized for a control panel or keyboard as illustrated, for example, in Figure 8. Additionally, as described in detail below, the force required to activate the switch or switches can vary across the surface of the switch by varying the pattern of the dielectric spacer layers 20. As Figure 3 illustrates, if desired, the flexible substrate 18 of at least one assembly, such as the assembly 14, can be replaced with a stiff material, such as a circuit board 29. This provides a surface against which the switch 10 can be depressed by a user. It is to be noted, however, that due to the unique pattern of the dielectric spacer layers 22, any stiff material is not necessary, especially with patterns requiring less actuation forces. The dielectric spacer layer 22 also is deposited with a uniform thickness on top of the conductive layer 20 such as by deposition, silk screening or any similar process where the thickness can vary. Preferably, the pre-determined pattern of the dielectric spacer layer 22 is in the form of a series of hexagons forming a plurality of substantially identical cells 30 across the surface of the conductive layer 20. Each cell 30 is defined by six cell walls 32 which are shared with adjacent cells in the pattern. It has been found that the hexagonal pattern provides the most conductive surface areas with the least amount of dead space at the wall junctions, as is found if the pattern is in the form of rectangles, circles or similar patterns. Each cell wall 32 is broken proximate its mid-point to provide channels 34 between adjacent cells 30. Such channels 34 enable air between the first and second assemblies 12 and 14 to move between one or more adjacent cells 30 when depressed to activate the switch 10. Additionally, when the switch 10 is released, the channels 34 enable air to re-enter the depressed cell or cells 30. This design completely eliminates any vacuum from forming between the cells 30 and the switch assemblies 12 and 14 which, if present, would prevent the switch 10 from breaking contact after the user depressed the switch 10. The channels 34 of each cell 30 preferably align with the channels 34 on the opposite side of the cells 30 so that three continuous sets of parallel lines or air channels are formed across the assemblies 12 and 14. These lines or air channels assist in the free flow of air between one or more cells 30 which in turn provides better actuation and release of the switch 10. In order to vary the force required to activate the switch 10, the size of the hexagons is adjusted. For example, Figure 4 illustrates a switch 10a having enlarged hexagons which require an actuation force F1 which is rather weak while the hexagon pattern still prevents sagging within the cells 30. Figure 5 illustrates a switch 10b having reduced hexagons which require an actuation force F2 which is stronger than force F1. Thus, the actuation force required can be adjusted to provide a variety of different switches, such as switches 10a or 10b, each of which have different actuation forces merely by changing the size of the hexagons. Additionally, a switch 10 can be provided having two or more different size hexagonal patterns as well as one or more individual switch areas therein. If one switch area is to be provided, different size hexagonal patterns can be positioned about the surface of the switch 10 to change the actuation force required in different areas. If two or more switch areas are to be provided, the size of the hexagonal pattern can be varied to provide the different switch areas with a desired actuation force as well as providing different actuation forces across the surface of any individual switch switch area. Figures 6 and 7 illustrate the basic operation of the switch 10. In Figure 6, a user contacts and depresses the flexible substrate 18 of the assembly 12, such as with a finger or other implement, in the direction of arrow "A". Upon being depressed, air is evacuated from one or more cells 30 through one or more channels 34 to one or more adjacent cells 30 and the two conductive layers 20 touch to complete the circuit and activate the switch. As Figure 7 illustrates, upon being released, the resiliency of the flexible substrates 18 and the dielectric spacer layers 22 force the assembly 12 upward in the direction of arrow "B". This enables air to de drawn back into the depressed cell or cells 30 to break contact with the two conductive layers 20 and provide an open circuit. Figures 9 and 10 illustrate the switch 10 utilized with an air bag assembly 40 which is illustrated installed in a steering wheel housing 42. The air bag assembly 40 typically includes and air bag 44 and an outer cover, skin or containment member 46 which is utilized to contain the air bag 44 within the housing 42 until deployment is desired. The switch 10 can be utilized in this type of environment preferably to provide a horn switch which typically is provided in the centre of the steering wheel. Since the switch 10 does not require a backer board, the switch can be provided directly in contact with the air bag 44. Due to the flexibility of the switch 10, depression of the switch 10 will not damage the air bag 44. Additionally, upon deployment of the air bar 44, the switch 10 merely bends out of the way without damaging the air bag 44 or causing injury to a user. It is to be understood that the switch 10 can be utilized in any housing, not just a steering wheel housing 42, and provided with numerous individual switch areas to enable the user to activate a variety of functions. Additionally, if desired, some type of backer board or substrate (not illustrated) can be utilized which can be hingedly mounted to the housing and would not cause damage to the air bag 44 or the user. Typically, such a backer board is not required unless large actuation forces are necessary.

Claims: A membrane switch comprising first and second substantially flexible electrical contact means (18, 20) and a first layer of dielectric material (22) deposited between and on one surface of one of said first or second contact means (18), said first layer (22) being deposited in a pre-determined geometric pattern including a plurality of cells (30) defined by cell walls (32), wherein said cell walls (32) are broken in pre-determined positions (34) about the perimeter of each cell (30) so that, during and after actuation of said switch, air within any cell (30) between said first and second contact means (18) can vent to one or more adjoining cells (30), characterised in that said geometric pattern includes a series of hexagons defining a plurality of cells (30) where each of the six sides of the hexagons are broken proximate a mid-point thereof to enable activation and prevent a vacuum from forming between said first and second contact means (18) thereby enabling said switch to break contact after being depressed. A membrane switch as defined in claim 1, wherein said first and second contact means are provided by depositing a conductive layer of material (18) on a respective first and second flexible substrate (20). A membrane switch as defined in claim 1 or 2, including a second layer of dielectric material (22) deposited on the surface of the other of said first and second contact means opposite said first layer (22), said second layer (22) being deposited in a pre-determined geometric pattern substantially identical to said first layer and wherein said cell walls (32) of said first and second layers are in alignment. A membrane switch as defined in any preceding claim, wherein said first and second electrical contact means are secured about their perimeters with an adhesive (16) to environmentally protect the switch contacts (18) by preventing infiltration of the elements there-between. A membrane switch as defined in any preceding claim, wherein a resilient member (46) is provided on an exterior surface of at least one of said first and second contact means (18, 20) to distribute both mechanical and electrical switch loads over more than one cell (30). An air bag assembly (40), comprising: housing means (42) having at least one opening therethrough; air bag means (44) mounted within said housing means (42); an outer cover member (46) enclosing said at least one opening in said housing (42) and securing said air bag means (44) therein, said outer cover (46) capable of enabling release of said air bag means (44) through said opening in said housing (42) when activated; and a membrane switch means (10) in accordance with any one of the preceding claims, mounted between said air bag means (44) and said outer cover (46) which can be activated by a user through said outer cover (46), enable unimpeded deployment of said air bag means (44) while preventing damage to said air bag means (44) and injury to a user.

Inventors: ASTA JOSEPH L, ASTA, JOSEPH L.
Applicants: ILLINOIS TOOL WORKS, ILLINOIS TOOL WORKS INC.
Application year: 1994
Main classification: H01H3
IPC codes: B60R21, H01H13, B60Q5, H01H3